It has become apparent in recent years that bacterial infections in hospitals, nursing homes, gyms, animal barns and other places where disease-causing bacteria can survive and spread have risen significantly. Furthermore, many strains of bacteria have become increasingly more resistant to available antibiotics that previously could be used to treat acquired infections. Thus, infections acquired in environments where bacteria thrive are and will continue to be serious health threats.
Efforts to control bacterial infection in places where such illnesses can spread, such as hospitals, are usually centered on destroying the organisms that cause infection with antimicrobial agents known as disinfectants or sanitizers.
Disinfectants, as defined by the Environmental Protection Agency (EPA), are liquid substances that can be sprayed or wiped on hard inanimate surfaces that result in the destruction or irreversible inactivation of infectious fungi and bacteria on the treated surface. Hospital disinfectants are most critical to infection control and are used on medical and dental instruments, floors and walls, bed linens, toilet seats and other surfaces.
Sanitizers are used to reduce, but not necessarily eliminate, the activity of microorganisms from inanimate surfaces to levels considered safe as determined by public health codes or regulations. Sanitizers include food contact products that are used on sites where consumable food products are placed or stored such as dishes and cooking utensils and equipment found in dairies, food processing plants and restaurants.
By late 2013, the EPA had registered about 275 active antimicrobial ingredients. The known antimicrobials can be classified into nine main categories: acids, alcohols, aldehydes, alkalis, biquanides, halogens, oxidizing agents, phenols and quaternary ammonium compounds.
Acidic disinfectants such as acetic acid, citric acid and fatty acids destroy the bonds of nucleic acids and precipitating proteins. Their effectiveness as antimicrobial agents is highly pH dependent, i.e. increased acidity enhances their effectiveness. Concentrated solutions of acids can be corrosive, cause chemical burns, and can be detrimental to the lungs and skin at high concentrations in the air. Fatty acids are less corrosive but can only be used in combination with a soap or surfactant since they are oily substances that are insoluble in water. Fatty acids also have highly offensive odors. These characteristics limit the use of acidic disinfectants.
Alcohols such as ethanol and isopropanol are broad-spectrum antimicrobial agents that damage microbes by denaturing proteins causing membrane damage and cell lysis. To be effective disinfectants, a high concentration, usually 65-90 weight percent, must be used. The activity of alcohols is limited in the presence of organic matter. Furthermore, alcohols are highly flammable, can cause damage to rubber and plastics, and can be very irritating to injured skin.
Aldehydes such as formaldehyde and glutaraldehyde are highly effective, broad-spectrum disinfectants that mainly achieve sterilization by denaturing proteins and disrupting nucleic acids. These substances are highly irritating, toxic to humans and animals on contact or inhalation and are potentially carcinogenic; therefore, their use is limited.
Alkali disinfectants, such as sodium or ammonium hydroxide, work by dissolving lipids within the envelopes of the microorganism. The activity of alkali compounds is slow but can be increased by increasing the treatment temperature. Sodium hydroxide is highly caustic and protective clothing, rubber gloves and safety glasses must be used during application. Ammonium hydroxide is very odorous and the vapors can be injurious to the eyes and lungs. Alkali disinfectants are also not considered to be effective against most bacteria.
Biguanides, as represented by chlorhexidine, kill microorganisms by reacting with the negatively charged groups on cell membranes. They can only function in a limited pH range (5-7) and are easily inactivated by soaps and detergents.
Halogen compounds such as sodium hypochlorite and iodine compounds are broad-spectrum antimicrobials that are inexpensive, readily available and easy to use. They function by denaturing proteins. Sodium hypochlorite can be very caustic, is irritating to the mucous membranes, eyes and skin, is rapidly deactivated by light and some metals, loses effectiveness in the presence of other organic matter and has poor residual activity. Iodine agents have the same advantages and disadvantages as sodium hypochlorite and are deactivated by quaternary ammonium compounds and especially organic debris.
Oxidizing agents such as hydrogen peroxide and peracetic acid function by denaturing the proteins and lipids of microorganisms. In their diluted form, these agents are relatively safe but may be irritating and damage clothing when concentrated. Some metals, e.g. iron, rapidly deactivate them. They also lose effectiveness when there is organic debris present.
Phenols (pine oils) are broad-spectrum disinfectants that function by denaturing proteins and inactivating membrane-bound enzymes to alter the cell wall permeability of microorganisms. Phenols are usually applied as water emulsions with soap or detergents since they have low solubility in water. Concentrations of 5% are most effective but they are not effective against microorganism spores. Prolonged exposure to the skin may cause irritation. Concentrations over 2% are highly toxic to all animals, especially cats.
Quaternary ammonium compounds, referred to as “quats” or QACs, are cationic detergents that are attracted to negatively charged surfaces of microorganisms where they irreversibly bind phospholipids in the cell membrane and denature proteins, thus impairing permeability. They are most active at neutral to slightly alkaline pH but lose their activity at a pH less than 3.5. Organic matter, detergents, soaps and hard water readily inactivate quats.
Each of the disinfectants referenced above has serious shortcomings that limit their use. Some are hazardous to apply and none of them are effective in all situations. While most of these disinfectants have been known for decades, there have been very few antimicrobials discovered in recent years.
In J. V. Karabinos and H. J. Ferlin, The Journal of the American Oil Chemists' Society, June 1954, Volume 31, Issue 6, pp 228-232, the antimicrobial activity of fatty acids in the C9 to C12 range in dilute acetic acid were examined. The authors concluded that the more acidic the solution, the higher the antibacterial effectiveness. In an article entitled Antimicrobial Agents Derived from Fatty Acids by J. J. Kabara, The Journal of the American Oil Chemists' Society, vol. 61, no. 2, pp. 397-403 (1984), the author screened even numbered fatty acids from C8 to C18 (both saturated and unsaturated) for antimicrobial activity against four different bacteria. He found no activity toward Pseudomonas aeruginiosa while the C10 acid (capric acid) gave the highest activity against Streptococcus Group A, Staphlycoccus aureous and Candida albicans of any of the fatty acids tested. The only fatty acid derivatives studied were esters.
U.S. Pat. No. 3,867,300 also discloses antimicrobial activity of fatty acids in water in the presence of non-ionic or anionic detergents at an optimal pH of 6 to 8. U.S. Pat. No. 4,404,040 describes short-chain fatty acid compositions with sanitizer solutions in the pH range of 2.0 to 5.0. U.S. Pat. No. 5,330,769 disclose the use of mixtures of nonanoic and decanoic acids (C9 and C10 fatty acids) in strong acids as sanitizers sufficient to lower the pH of the final solutions to 1 to 5. In addition to low pH, these references require water solubilizers or emulsifiers in the formulations with the fatty acids in order to allow the acids to be dispensed in water.
Accordingly, there is a need for alternative disinfectants effective against microorganisms, are safe to use without protective gear (other than eye protection) during their use, are non-toxic to humans or animals by ingestion or absorption through the skin and are relatively low cost.